Heretofore, a disc monopole antenna, which is disclosed in M. Hammoud et al, “Matching The Input Impedance of A Broadband Disc Monopole”, Electron. Lett., Vol. 29, No. 4, pp. 406–407, 1993, has been known as an antenna having an operating frequency band in a wide band. FIG. 31 is a schematic view showing this disc monopole antenna. This disc monopole antenna is configured to include a planar monopole 101 connected to a coaxial line 102. Specifically, the planar monopole 101 is disposed as to be upright with respect to a metal plate 103 at a position away from the metal plate 103 by a distance L. It is possible to provide optimum matching so as to have a desired characteristic by adjusting the distance L.
Additionally, an antenna, which is shown in FIG. 32 and is disclosed in Japanese Patent No. 3,114,798, has been known. This antenna includes a planar monopole 105, which is upright from a metal plate 103. The planar monopole 105 is a monopole, which has such a planar structure to have the transverse width of a disc shape (circular shape) reduced so as to have a tapered shape. This antenna forms a monopole antenna having an operating frequency band adapted for a wide band by using the planar monopole 105, an unshown corner reflector and the metal plate 103. The corner plate has a structure wherein two planar plates having certain dimensions have edges bonded together, and the bonded portion is bent in a dogleg shape. The corner reflector is disposed so as to be perpendicular to the metal plate 103 and have two bonded plates extending orthogonally with each other. The tapered planar monopole 105 has a lower portion formed with a linearly cut-out portion 106 so that the distance between the metal plate 103 and an edge of the tapered planar monopole 105 close thereto is set at a required length L.
Sung-Bae Cho et.al., “ULTRA WIDEBAND PLANAR STEPPED-FAT DIPOLE ANTENNA FOR HIGH RESOLUTION IMPULSE RADAR”, 2003 Asia-Pacific Microwave Conference, discloses another planar dipole antenna, which has an operating frequency band in a wide band. This planar antenna has a structure wherein a pair of metal conductors having a similar shape, which serves as a radiating conductor, is disposed on a dielectric member so as to be separated from each other with a certain distance, and power is fed to the paired metal conductors from a region between the separated conductors.
Each of the antenna devices shown in FIG. 31 and FIG. 32 uses a monopole antenna. Each of the antennas is configured to include a radiating element comprising the planar disc monopole 101 or the planar monopole 105 and the metal plate 103. The radiating element and a ground conductor are disposed so as to be perpendicular and orthogonal with each other. Accordingly, the radiating element is disposed to be upright with respect to the ground conductor so as to have a three-dimensional configuration, occupying a three-dimensional space as an antenna having a three-dimensional structure. In the antenna shown in FIG. 31, the metal plate 103 has a large shape having, e.g., 300 mm×300 mm since the metal plate needs to have a size, which is about 10 times the diameter of the planar disc monopole 101. On the other hand, in the antenna device shown in FIG. 32, the antenna and the unshown corner reflector are disposed so as to be perpendicular with respect to the ground conductor. Accordingly, the antenna and the corner reflector are disposed to be upright with respect to the ground conductor so as to have a three-dimensional configuration, occupying a three-dimensional space as a three-dimensionally configured antenna device.
The antennas shown in FIG. 31 and FIG. 32 are not suited for a small size antenna since both antennas are formed in a three-dimensional structure and have a large shape.
Additionally, the antenna device shown in FIG. 32 provides good impedance matching with respect to different frequencies by forming the linearly cut-out portion having a width of about 1 to 2 mm in the tapered planar monopole 105 having a length of 36 mm for instance. However, the operating frequency band is not always in a sufficiently wide band since the radiating conductor comprising the planar monopole 105 has a tapered shape, which is determined in accordance with the dimensions of the reflector stated earlier. For example, the operating frequency band has only a fractional bandwidth of 33%, explanation of a fractional bandwidth being described later.
Although the planar dipole antenna disclosed in the second non-patent document has an operating frequency band in a wide band, this planar antenna is not an antenna having a high degree of freedom in design since the paired metal conductors forming a radiating element need to have a stepped shape.